1. Field of the Invention
This invention relates to an optical barrel designed as an optical projection unit, such as a projection unit for a projection exposure apparatus used in a semi-conductor wafer production. More particularly, the present invention relates to a hybrid optical barrel having a sub-barrel and cells being made of different materials.
2. Description of the Related Art
An optical barrel is a tubular mechanical structure including a sturdy threaded body and durable metal retaining rings, often referred to as lock rings, to retain a series of lens holders, commonly referred to as cells. A lens is to be mounted in a cell. This optical barrel design applies in many optical systems. For example, the optical barrel may be used for attachment to an optical projection unit as a part of a projection exposure apparatus used in production of semiconductor wafers. See, for example, U.S. Pat. No. 5,528,118 to Lee entitled "Guideless Stage With Isolated Reaction Stage."
The projection exposure apparatus reduces a pattern on a template, such as a reticle or a mask, through the optical projection unit and sequentially transfers the pattern onto a predetermined area on a semiconductor wafer or plate, which is a photo-sensitive substrate for making computer chips, such as microprocessor or memory chips. A focus depth of the optical projection unit becomes very small as the miniaturization of a circuit pattern progresses. Therefore, a primary consideration for this type of optical barrel structure includes designing an optical barrel capable of having high tolerances for holding the barrel components for both stability and accuracy. In addition, the optical barrel preferably transfers a pattern that can be focused on the photosensitive substrate over a wide temperature range. However, coefficients of thermal expansion of barrel components, as well as indices of refraction of all lens materials, vary with temperature which collectively alter the focusing and distortion properties of the optical system. Many have developed optical designs and manufacturing processes to minimize this effect, but a common problem of the applications has originated from this temperature dependence.
Two conventional optical barrel structures are currently available in the market. One structure involves a barrel carrying a plurality of cells (also known as "lens holders") on which lenses are mounted. The cells slidably fit inside the barrel. This structure is commonly referred to as the conventional barrel type. Another structure involves a barrel comprising a series of sub-barrels connected end to end by fasteners to construct the barrel itself. This structure is referred to as the sub-barrel type. In both types of the conventional optical barrels, the barrel and the sub-barrels are commonly made of brass because brass is relatively inexpensive and easy to machine to achieve components with high tolerances. However, one of the disadvantages of brass barrel constructions is that they are not very stiff and are easily affected by temperature changes.
In light of the foregoing, there is a need for a stiffer optical barrel structure that can withstand temperature changes and remain dimensionally stable over a wide temperature range.